In recent years, in the field of refrigerators, refrigerants which have heretofore been used and have great ozonosphere destruction potential due to the contained chlorine such as chlorofluorocarbons and hydrochlorofluorocarbons are being replaced with hydrofluorocarbon-based refrigerants having the ozonosphere destruction potential of zero from the standpoint of global ozonosphere protection. At the same time, so-called natural substance-based refrigerants such as carbon dioxide, ammonia and hydrocarbons having small global warming potential are attracting attention from the standpoint of prevention of the global warming.
When the hydrofluorocarbon-based refrigerant or the natural substance-based refrigerant is used for a refrigerator, the application of mineral oil-based lubricating oils which have heretofore been used becomes difficult from the standpoint of miscibility. As the result, lubricating oils using an oil having oxygen such as a polyalkylene glycol-based oil, a polyvinyl ether-based oil, a polyol ester-based oil and a polycarbonate-based oil as the base oil are used in combination with the above refrigerants. In particular, the polyalkylene glycol-based lubricating oil for refrigerators are used as the lubricating oil for refrigerators of automobile air conditioners due to the great viscosity index.